The present invention relates to a display apparatus including a CRT.
FIG. 9 shows a structure of a conventional CRT display apparatus. In the figure, there is shown a CRT 18, a cathode 2, a G1 electrode 3, a G2 electrode 4, a G3 electrode 6, an anode 7, a video circuit 9, a flyback transformer (FBT) 12, an anode current measuring circuit 13, a resistor 14, a capacitor 15, and a variable resistor 19. The G1 electrode 3, G2 electrode 4, and G3 electrode 6 are cylindrical-shaped electrodes disposed within an electron gun to draw electrons from the cathode 2 and converge them. Other focusing electrodes disposed after the G3 electrode are omitted from the drawing to simplify explanation.
The operation of the apparatus of FIG. 9 will now be explained. A video signal is amplified in the video circuit 9, and supplied to the cathode 2. A high tension produced by the FBT12 is applied to the anode 7. The G2 electrode 4 is applied with a voltage obtained by dividing the high tension by the resistor 19. The FBT12 is supplied with a current from the resistor 14 within the anode current measuring circuit 13, and the capacitor 15 is charged at this time. It is possible to determine the anode current from the value of a voltage drop caused by the current flowing through the resistor 14. The value of this voltage drop is supplied to the video circuit 9.
The high tension of about 25 kV applied to the anode 7 is obtained by stepping up horizontal flyback pulses produced by a horizontal deflection circuit (not shown) and rectifying them by the FBT 12. The voltage of about 700 to 1000V applied to the G2 electrode 4 is produced by dividing this high tension by the resistor 19. Since the current flowing through the G2 electrode 4 is very small, the resistor 19 for dividing the high tension has a resistance as much as about 100 Mohm. A screen adjustment (coarse cutoff adjustment) can be performed to change a black level by adjusting the voltage applied to the G2 electrode 4.
Such a CRT display apparatus is usually provided with an automatic contrast limiting (ACL) circuit (also called an automatic brightness limiting (ABL) circuit), in order to prevent an average electron beam flowing from the cathode to the screen from exceeding an allowable level. Since the anode current is in proportion to the current of an electron beam (referred to as a xe2x80x9cbeam currentxe2x80x9d hereinafter), it is possible to determine the value of the beam current by measuring the anode current flowing through the FBT 12. The measured value of the anode current is supplied to the ACL circuit. Various types of anode current measuring circuit can be used. In the apparatus of FIG. 9, the anode current is measured from the value of the voltage drop across the resistor 14 caused by the current flowing therethrough. The value of this voltage drop is supplied to the video circuit 9 which includes a preamplifier, an image-enhancement circuit, etc. When the anode current exceeds the allowable level, the video circuit 9 suppresses the amplitude of the video signal supplied to the cathode by reducing its amplification factor of the video signal. Consequently, the beam current is suppressed and the intensity is reduced.
On the other hand, the demand for improving resolution of CRT display apparatuses is growing in recent years. Japanese Unexamined Patent Publication No. 11-224618 discloses a-high intensity/resolution CRT (referred to as xe2x80x9cHi-Gm tubexe2x80x9d hereinafter) that addresses such a demand. This Hi-Gm tube features a novel electron gun that has, in addition to the G1, G2 and G3 electrodes, an electrode called xe2x80x9cGm electrodexe2x80x9d disposed between the G2 electrode and the G3 electrode for modulating the electron beam.
FIG. 10 shows a structure of such an electron gun used for the Hi-Gm tube. In this figure, 20 denotes a G1 electrode, 21 denotes a G2 electrode, 23 denotes a cathode, 24 denotes an electron-emitting substance formed on the surface of the cathode 23, and 25 denotes a Gm electrode. This electron gun has, for the part following the G3 electrode where other focusing electrodes are disposed, the same structure as the conventional electron gun.
FIG. 11 is a graph showing potential distribution near the cathode within the electron gun of the Hi-Gm tube. In this graph, the horizontal axis represents the distance (mm) from the cathode surface, the vertical axis represents the potential (V), and the curve 26 shows the potential distribution symmetrical with the axis of revolution near the cathode. Furthermore, the arrow 27 shows the range within which the Gm electrode 25 exists, which is about 0.5 mm from the cathode surface.
The potential of the Gm electrode 25 is set to about 80VDC, so there is a position 28 within the range 27, at which the level of the spatial, potential is minimum. If the potential of the cathode 23 shown by the dashed line is lower than the potential at this position 28, electrons pass through the position 28 and flow towards the screen. If not, electrons do not flow towards the screen since they cannot pass through the position 28.
As seen from this graph, between the cathode 23 and the position 28, electrons always exist abundantly, and the slope of the potential after the Gm electrode 25 is of the order of 106 (V/m). Compared with the potential slope between the cathode and the G1 electrode, it is greater by an order of magnitude. Therefore, after electrons pass through the Gm electrode 25, most of them can move towards the screen without being affected by spatial charges, so the intensity of the electron beam flowing to the screen is determined by the quantity of the electrons that pass through the position 28 at which the spatial potential is minimum.
For this reason, variation of the intensity of the electron beam in the Hi-Gm tube when the cathode potential is varied by a certain value in the Hi-Gm tube is about twice as much as that in the conventional CRT. That is, the variation of the cathode potential required to vary the intensity of the electron beam by a certain value is less than half the variation required in the conventional CRT. In other words, with the Hi-Gm tube, the variation of the intensity of the electron beam can be doubled for the same variation of the cathode potential. Consequently, with the Hi-Gm tube, it is possible to easily adapt to video signals of high frequency, and therefore to provide a display apparatus of high intensity and high resolution.
FIG. 12 is a graph showing how the cathode current, the beam current, the G2 electrode current, and the Gm electrode current vary when the cathode voltage varies. In this graph, reference numeral 29 denotes the cathode current, 30 denotes the beam current, 31 denotes the G2 electrode current, and 32 denotes the Gm electrode current. This graph holds while the G2 electrode voltage is 500V, and the Gm electrode voltage is 80V. From this graph, it is apparent that as the cathode voltage decreases, the beam current increases and thereby the brightness of the screen is enhanced, and that the beam current starts to flow towards the screen when the cathode voltage falls below 80V, since the voltage applied to the Gm electrode is 80V. Furthermore, it is also apparent form this graph that the Gm electrode current and the G2 electrode current increase as the beam current increases.
In the display apparatus using the above-described Hi-Gm tube, since the variation of the beam current can be more than twice the variation in the case of a CRT display apparatus using the conventional electron gun for the same variation of the cathode voltage, the possibility of the beam current becoming excessive is higher for that. If the excessive beam current continues to flow, emission failure etc. can occur which leads to shorten a CRT lifespan. Therefore, in the display apparatus using the Hi-Gm tube, the control over the beam current is more important than ever before. An object of the present invention is to provide a CRT display apparatus provided with a novel structure for preventing its beam current from becoming excessive in consideration of the above-described characteristic of the Hi-Gm tube.
The object is achieved by a CRT display apparatus including a CRT having an electron gun,
the electron gun including:
a cathode;
a G1 electrode, a G2 electrode, and a G3 electrode disposed in that order for drawing electrons form the cathode; and
a modulating Gm electrode disposed between the G2 electrode and the G3 electrode;
the CRT display apparatus further including:
a current measuring circuit for measuring one of a current flowing through the Gm electrode, a current flowing through the G2 electrode and a current flowing through an anode of the CRT; and
a controller for controlling a value of a voltage applied to the Gm electrode according to a value of the current measured by the current measuring circuit.
The object is also achieved by a CRT display apparatus including a CRT having an electron gun,
the electron gun including:
a cathode;
a G1 electrode, a G2 electrode, and a G3 electrode disposed in that order for drawing electrons form the cathode; and
a modulating Gm electrode disposed between the G2 electrode and the G3 electrode;
the CRT display apparatus further including:
a current measuring circuit for measuring one of a current flowing through the Gm electrode, a current flowing through the G2 electrode and a current flowing through an anode of the CRT; and
a controller for controlling a value of a voltage applied to the G2 electrode according to a value of the current measured by the current measuring circuit.
The object is also achieved by a CRT display apparatus including a video circuit and a CRT having an electron gun,
the electron gun including:
a cathode;
a G1 electrode, a G2 electrode, and a G3 electrode disposed in that order for drawing electrons form the cathode; and
a modulating Gm electrode disposed between the G2 electrode and the G3 electrode;
the video circuit supplying a video signal having an amplitude determined by a control signal to the cathode,
the CRT display apparatus further including:
a current measuring circuit for measuring a value of one of a current flowing through the Gm electrode, a current flowing through the G2 electrode and a current flowing through an anode of the CRT, and supplying the measured value to the video circuit as the control signal.